THE FUNCTIONAL-ANATOMY OF SOUND INTENSITY DISCRIMINATION

The human neuroanatomical substrate of sound intensity discrimination was investigated by combining psychoacoustics and functional neuroimag ing. Seven normal subjects were trained to detect deviant sounds prese nted with a slightly higher intensity than a standard harmonic sound, using a Go/No Go paradigm. Individual psychometric curves were careful ly assessed using a three-step psychoacoustic procedure. Subjects were scanned while passively listening to the standard sound and while dis criminating changes in sound intensity at four different performance l evels (d' = 1.5, 2.5, 3.5, and 4.5). Analysis of regional cerebral blo od flow data outlined activation, during the discrimination conditions , of a right hemispheric frontoparietal network already reported in ot her studies of selective or sustained attention to sensory input, and in which activity appeared inversely proportional to intensity discrim inability. Conversely, a right posterior temporal region included in s econdary auditory cortex was activated during discrimination of sound intensity independently of performance level. These findings suggest t hat discrimination of sound intensity involves two different cortical networks: a supramodal right frontoparietal network responsible for al location of sensory attentional resources, and a region of secondary a uditory cortex specifically involved in sensory computation of sound i ntensity differences.